Carbide pellets may generally be used for wear resistant applications, such as composite materials for forming bits, for example drill bits for earth-boring drills, or as hard facing compositions, for example, hard facing compositions for rock bits or as a plasma tungsten arc coating compositions. When used in hard facing applications, the carbide pellets are generally cemented or sintered tungsten carbide pellets.
U.S. Pat. No. 4,944,774 to Keshavan et al. discloses cemented tungsten carbide used in hard facing materials. The cemented tungsten carbide comprises small particles of tungsten carbide bonded together with cobalt in amounts ranging from 6 to 8 weight percent. The cemented tungsten carbide is made by mixing tungsten carbide, organic wax, and cobalt powders; pressing the mixed powders to form a green compact; and sintering the composite at temperatures near the melting point of cobalt. The resulting dense cemented carbide can then be comminuted to form particles of cemented tungsten carbide for use in hard facing applications. Other hard facing compositions are disclosed in U.S. Pat. Nos. 3,800,891; RE37,127; 6,248,149; 6,659,206; and 6,782,958.
Pan and tube granulation processes have conventionally been used to make carbide pellets containing relatively large amounts of metallic binder, e.g., 6 weight percent cobalt. In these techniques, tungsten carbide powder and cobalt powder are milled with wax in an organic solution for several hours, then the milled powder is dried in a vacuum dryer.
In the pan granulation process, the powder is fed continuously to the top of a rotating disk pelletizer to form green pellets. The disk pelletizer typically rotates at approximately 15 revolutions per minute at an angle of 50° to 75° relative to the horizontal plane. Agglomeration occurs by particle coalescence as the pelletizer rotates. The larger agglomerates rotate to the outer pan rim and are readily discharged from the pan.
In the tube granulation process, the milled and dried powder is fed into a tube or drum pelletizer at one end to form green pellets. The drum pelletizer rotates at approximately 15 revolutions per minute to cause agglomeration by particle coalescence. The agglomerates are continuously discharged at the other end of the tube.
In both the pan and tube granulation processes, the agglomerated green pellets may be sized. Undersized pellets may be recycled, and oversized pellets may be crushed and recycled, by feeding the pellets back to the granulator with the powders. The properly sized green pellets are then sintered, and may be broken into individual pellets if necessary.
While pan and tube granulation processes have effectively been used to make carbide pellets with relatively large amounts of metallic binder, attempts to make carbide pellets containing less than 3 weight percent cobalt by such processes have been unsuccessful. The present invention provides an improved process for forming carbide pellets having a metallic binder, such as cobalt, in an amount less than 3 weight percent.